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Bill S.
Guest
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| ADC -- Potentiometer that measures tilt angle |
 Posted: Fri Nov 11, 2005 4:37 pm |
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Hello,
Was wondering if you guys can help me out.
I have a potentiometer on a pendulum pole (placed straight above the car's roof) that will detect my R/C car's tilt angle. Input voltage to the 10K linear potentiometer is 5V. At rest the voltage is at 2.5V. My range will be from 1.5V - 3.5 V depending oh how far my pendulum swings.
When the voltage changes I would like to figure out the angle of the car based on the voltage change (eg. 10 degrees left or right ). Is there a way to create a "lookup" table to quickly determine the angle (put into a variable called angle) when a certain voltage is applied?
So far I got this: (Used http://mdubuc.freeshell.org/Common/gp2d12.c as an example...... unless there is another way for a lookup table.
I will manually figure out the angles (left and right) based on different voltages I apply )
| Code: | /*Tilt angle interface */
uint8_t get_tiltangle(uint8_t tiltangle);
void monitor_tiltangle_sensor(void);
/* Variable definitions */
???
???
static const uint8_t tilt_calib_data[NUM_TILT_CALIB_DATA] = {
124, 107, 93, 83, 75, 67, 62, 57, 53, 49, 47, 44, 42, 39, 38, 36,
34, 33, 32, 30, 29, 28, 27, 26, 25, 25, 24, 23, 23, 22, 21 }
???
???
};
/****************************************************/
void main (void) {
for ( ; ; ) {
monitor_tiltangle_sensor();
display_tiltangle();
}
}
/****************************************************/
/****************************************************/
uint8_t get_tiltangle(uint8_t tiltreading) {
int i;
??
??
??
??
} /* get_tiltangle */
/****************************************************/
/****************************************************/
void monitor_tiltangle_sensor(void) {
setup_adc_ports(A_ANALOG);
setup_adc(ADC_CLOCK_DIV_8);
set_adc_channel(ANGLE_ANCH);
value = Read_ADC();
setup_adc(ADC_OFF);
angle = get_tiltangle(value);
}
/****************************************************/ |
What I'm basically doing is if the angle is above a certain threshold say 20 degrees...I will slow down the RC vehicle so it will not rollover (pole is approx 3m high , so it will flip over if the car is travelling fast).
Also, would I just be needing one input port on the PIC controller to determine the voltages of the poteniometer?? eg. RA2.
THank you, |
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kender
Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley
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| Posted: Fri Nov 11, 2005 9:58 pm |
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You are correct - you need only one analog input on the pic to read the pot. The resistance of your pot is too high though. The PIC's datasheet asks for the output impedance less then 2.5k. With 10k pot the input impedance will be in the neiborhoud of 5k. I would recommend getting a pot with R < 2.5k or buffering your signal with an op-amp.
I think you have a good example of a lookup table. However, I don't think you need a lookup table, because you don't need to use trigonometric functions.
Here's the schematic, which shows your pot as 2 resistors:
| Code: |
Vcc
|
\
/
\ R1 = a * r
/
\
/ wiper of your pot
|<------------------------------->>V to the analog input of the PIC
\
/
\ R2 = (p - a) * r = R - R1
/
\
/
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///
GND
Where:
V voltage observed by the A/D [V] edit: or [a/d counts]
Vcc PIC's supply voltage [V] or 255 A/D counts or 1024 A/D counts
p total angular swing of your pot [deg]
R = R1 + R2 total resistance of your pot [ohm]
a angle of your pendulum meaured from the extreme position of the pot [deg]
r = R/p is resistance of your pot per degree [ohm/deg]
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For a resistive divider we can write:
V/Vcc = R2/(R1+R2) = a*r/R
Trivially, the angle is
a = (v/Vcc)(R/r) |
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Bill S.
Guest
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| Posted: Mon Nov 14, 2005 6:21 pm |
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Thank you for the reply. I did the following:
| Code: | void
monitor_angle_sensor(void) {
/* Read ADC */
setup_adc_ports(ALL_ANALOG);
setup_adc(ADC_CLOCK_INTERNAL);
set_adc_channel(ANGLE_CHANNEL); /* Read Port A1/AN1 */
delay_us(ADC_DELAY);
angle = Read_ADC();
} |
This gave me a number from 0 (V) - 1024 (5V)
At rest, my potentiometer ouptuts around 2.5V so I got a number of 508.. when I swing either left or right.. i get a range from 430 - 540. |
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kender
Joined: 09 Aug 2004
Posts: 768
Location: Silicon Valley
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| Posted: Mon Nov 14, 2005 6:44 pm |
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| Bill S. wrote: |
| Code: |
setup_adc_ports(ALL_ANALOG);
setup_adc(ADC_CLOCK_INTERNAL);
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Actually, you don't need to call these 2 functions every time you make an A/D conversion. You can call them just once in the main(), unless the ADC ports and clock are changing. |
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